Image forming apparatus

Abstract

An image forming apparatus may include a transfer nip, a first conveyance belt, a second conveyance belt, a fixing nip, a fixing drive unit, and a control unit. The first conveyance belt conveys the sheet to which a toner image is transferred and fixed. The fixing drive unit drives the fixing nip so that the fixing nip conveys the sheet at a speed that is higher than the transfer speed in a high speed fixing mode in which the fixing nip fixes the toner image to the sheet while conveying the sheet. The control unit sets a circumferential speed of the first conveyance belt and a circumferential speed of the second conveyance belt so that the sheet bridging the first and second conveyance belts is pulled by the second conveyance belt to slide on the first conveyance belt in the high speed fixing mode.

What is claimed is: 
     
       1. An image forming apparatus comprising:
 a transfer nip configured to transfer a toner image to a sheet conveyed at a transfer speed; 
 a first conveyance belt configured to convey the sheet to which the toner image is transferred at the transfer nip; 
 a second conveyance belt provided downstream of the first conveyance belt in a conveyance direction in which the sheet is conveyed and configured to convey the sheet; 
 a fixing nip provided downstream of the second conveyance belt in the conveyance direction and configured to fix the toner image to the sheet while conveying the sheet; 
 a fixing drive unit configured to drive the fixing nip so that the fixing nip conveys the sheet at a speed that is higher than the transfer speed in a high speed fixing mode in which the fixing nip fixes the toner image to the sheet while conveying the sheet at the speed that is higher than the transfer speed; and 
 a control unit configured to perform control so that a circumferential speed of the second conveyance belt is changed from a first speed at which the sheet advances from the transfer nip to a second speed at which a leading edge of the sheet enters the fixing nip and which is higher than the first speed after a trailing edge of the sheet, conveyed by the first conveyance belt whose circumferential speed is set to the first speed and the second conveyance belt whose circumferential speed is set to the first speed, passes the transfer nip and before the leading edge of the sheet reaches the fixing nip in the high speed fixing mode and in response to a length of the conveyed sheet in the conveyance direction being longer than a length from the transfer nip to an upstream end of the second conveyance belt, 
 wherein, after the circumferential speed of the second conveyance belt is changed to the second speed, the sheet sucked against the second conveyance belt is conveyed by the second conveyance belt while sliding on the first conveyance belt. 
 
     
     
       2. The image forming apparatus according to  claim 1 , wherein, in the high speed fixing mode, the leading edge of a succeeding sheet advancing from the transfer nip in succession to a preceding sheet reaches the first conveyance belt with the preceding sheet being on the first and second conveyance belts. 
     
     
       3. The image forming apparatus according to  claim 2 , wherein, in the high speed fixing mode, the control unit changes the circumferential speed of the second conveyance belt to the first speed after a leading edge of the preceding sheet conveyed by the second conveyance belt reaches the fixing nip, when the preceding sheet lies on the second conveyance belt, and before the succeeding sheet reaches the second conveyance belt. 
     
     
       4. The image forming apparatus according to  claim 1 , further comprising:
 a first suction device configured to suck the sheet against a circumferential surface of the first conveyance belt; and 
 a second suction device configured to suck the sheet against a circumferential surface of the second conveyance belt, 
 wherein a force by which the second suction device sucks the sheet against the circumferential surface of the second conveyance belt is made greater than a force by which the first suction device sucks the sheet against the circumferential surface of the first conveyance belt to cause the second conveyance belt to pull and convey the sheet so that the sheet slides on the first conveyance belt when the leading edge of the sheet reaches the fixing nip. 
 
     
     
       5. The image forming apparatus according to  claim 1 , further comprising:
 a first suction device configured to suck the sheet against a circumferential surface of the first conveyance belt; and 
 a second suction device configured to suck the sheet against a circumferential surface of the second conveyance belt, 
 wherein a friction coefficient of the circumferential surface of the second conveyance belt is made greater than a friction coefficient of the circumferential surface of the first conveyance belt to cause the second conveyance belt against which the sheet is sucked by the second suction device to pull and convey the sheet so that the sheet slides on the first conveyance belt against which the sheet is sucked by the first suction device when the leading edge of the sheet reaches the fixing nip. 
 
     
     
       6. The image forming apparatus according to  claim 1 , wherein, in response to a length of the conveyed sheet in the conveyance direction being shorter than a length from the transfer nip to an upstream end of the second conveyance belt and shorter than a length from a downstream end of the first conveyance belt to the fixing nip, the sheet advancing from the transfer nip to the fixing nip is conveyed with the circumferential speed of the first conveyance belt set to the first speed and the circumferential speed of the second conveyance belt set to the second speed and the sheet lying on the first and second conveyance belts is pulled and conveyed by the second conveyance belt while sliding on the first conveyance belt at a speed that is higher than the first speed. 
     
     
       7. The image forming apparatus according to  claim 1 , wherein, in the high speed fixing mode and in response to a length of the conveyed sheet in the conveyance direction being longer than a length from the transfer nip to an upstream end of the second conveyance belt and longer than a length from a downstream end of the first conveyance belt to the fixing nip, the control unit performs control so that the circumferential speed of the second conveyance belt is changed to the second speed after a trailing edge of the sheet, conveyed by the first conveyance belt whose circumferential speed is set to the first speed and the second conveyance belt whose circumferential speed is set to the first speed, passes the transfer nip and before the leading edge of the sheet reaches the fixing nip, and,
 after the circumferential speed of the second conveyance belt is changed to the second speed, the leading edge of the sheet reaches the fixing nip with the sheet sucked against the second conveyance belt pulled and conveyed by the second conveyance belt while sliding on the first conveyance belt. 
 
     
     
       8. An image forming apparatus comprising:
 a transfer nip configured to transfer a toner image to a sheet conveyed at a transfer speed; 
 a first conveyance belt configured to convey the sheet to which the toner image is transferred at the transfer nip; 
 a second conveyance belt provided downstream of the first conveyance belt in a conveyance direction in which the sheet is conveyed and configured to convey the sheet; 
 a fixing nip provided downstream of the second conveyance belt in the conveyance direction and configured to fix the toner image to the sheet while conveying the sheet; 
 a drive unit configured to drive the fixing nip so that the fixing nip conveys the sheet at a speed that is higher than the transfer speed in a high speed fixing mode in which the fixing nip fixes the toner image to the sheet while conveying the sheet at the speed that is higher than the transfer speed; 
 a suction unit provided for each of the first and second conveyance belts and configured to suck the sheet; and 
 a control unit configured to control an operation of the suction unit for sucking the sheet against each of the first and second conveyance belts, 
 wherein, in the high speed fixing mode and in response to a length of the conveyed sheet in the conveyance direction being longer than a length from the transfer nip to an upstream end of the second conveyance belt, a circumferential speed of the first conveyance belt is set to a first speed at which the sheet advances from the transfer nip and a circumferential speed of the second conveyance belt is set to a second speed at which a leading edge of the sheet enters the fixing nip and which is higher than the first speed and the control unit controls the suction unit so that the sheet is sucked against the first conveyance belt and slides on the second conveyance belt in a state that the sheet to which the toner image is transferred by the transfer nip is being conveyed by the first conveyance belt and controls the suction unit to cause the second conveyance belt to pull and convey the sheet so that the sheet slides on the first conveyance belt after a trailing edge of the sheet, conveyed by the first conveyance belt whose circumferential speed is set to the first speed, passes the transfer nip. 
 
     
     
       9. The image forming apparatus according to  claim 8 , wherein, in the high speed fixing mode, the control unit controls the suction unit so that the sheet is sucked against the first conveyance belt and the sheet is not sucked against the second conveyance belt until a trailing edge of the conveyed sheet passes the transfer nip, and controls the suction unit so that the sheet is sucked against the second conveyance belt and the sheet is not sucked against the first conveyance belt after the trailing edge of the conveyed sheet passes the transfer nip and before the leading edge of the sheet reaches the fixing nip. 
     
     
       10. The image forming apparatus according to  claim 9 , wherein the control unit controls the operation of the suction unit so that the sheet is not sucked against the second conveyance belt when the leading edge of a preceding sheet reaches the fixing nip and lies on the second conveyance belt and a succeeding sheet conveyed in succession to the preceding sheet is conveyed onto the second conveyance belt by the first conveyance belt while the preceding sheet is conveyed by the fixing nip and after the preceding sheet is not sucked against the second conveyance belt. 
     
     
       11. The image forming apparatus according to  claim 8 ,
 wherein the suction unit includes a first suction device configured to suck the sheet against the first conveyance belt and a second suction device configured to suck the sheet against the second conveyance belt, 
 wherein the first suction device includes a first fan and a first shutter configured to open and close between the first fan and the first conveyance belt, 
 wherein the second suction device includes a second fan and a second shutter configured to open and close between the second fan and the second conveyance belt, and 
 wherein the control unit opens and closes the first and second shutters to control the operation of the suction unit for sucking the sheet against the first and second conveyance belts. 
 
     
     
       12. The image forming apparatus according to  claim 8 , wherein, in the high speed fixing mode and in response to a length of the conveyed sheet in the conveyance direction being longer than a length from the transfer nip to an upstream end of the second conveyance belt and longer than a length from a downstream end of the first conveyance belt to the fixing nip, the circumferential speed of the first conveyance belt is set to the first speed and the circumferential speed of the second conveyance belt is set to the second speed and the control unit controls the suction unit so that the sheet is sucked against the first conveyance belt and slides on the second conveyance belt in a state that the sheet to which the toner image is transferred by the transfer nip is being conveyed by the first conveyance belt and controls the suction unit to cause the second conveyance belt to pull and convey the sheet so that the sheet slides on the first conveyance belt when the leading edge of the sheet reaches the fixing nip. 
     
     
       13. An image forming apparatus comprising:
 a transfer nip configured to transfer a toner image to a sheet conveyed at a transfer speed; 
 a first conveyance belt configured to convey the sheet to which the toner image is transferred at the transfer nip; 
 a second conveyance belt provided downstream of the first conveyance belt in a conveyance direction in which the sheet is conveyed and configured to convey the sheet; 
 a fixing nip provided downstream of the second conveyance belt in the conveyance direction and configured to fix the toner image to the sheet while conveying the sheet; and 
 a control unit configured to perform control so that a circumferential speed of the second conveyance belt is changed from a first speed at which the sheet advances from the transfer nip to a second speed at which a leading edge of the sheet enters the fixing nip and which is higher than the first speed after a trailing edge of the sheet, conveyed by the first conveyance belt whose circumferential speed is set to the first speed and the second conveyance belt whose circumferential speed is set to the first speed, passes the transfer nip and before the leading edge of the sheet reaches the fixing nip in response to a length of the conveyed sheet in the conveyance direction being longer than a length from the transfer nip to an upstream end of the second conveyance belt, 
 wherein, after the circumferential speed of the second conveyance belt is changed to the second speed, the sheet sucked against the second conveyance belt is conveyed by the second conveyance belt while sliding on the first conveyance belt. 
 
     
     
       14. The image forming apparatus according to  claim 13 , wherein the leading edge of a succeeding sheet advancing from the transfer nip in succession to a preceding sheet reaches the first conveyance belt with the preceding sheet being on the first and second conveyance belts. 
     
     
       15. The image forming apparatus according to  claim 13 , further comprising:
 a first suction device configured to suck the sheet against a circumferential surface of the first conveyance belt; and 
 a second suction device configured to suck the sheet against a circumferential surface of the second conveyance belt, 
 wherein a force by which the second suction device sucks the sheet against the circumferential surface of the second conveyance belt is made greater than a force by which the first suction device sucks the sheet against the circumferential surface of the first conveyance belt to cause the second conveyance belt to pull and convey the sheet so that the sheet slides on the first conveyance belt when the leading edge of the sheet reaches the fixing nip. 
 
     
     
       16. The image forming apparatus according to  claim 13 , further comprising:
 a first suction device configured to suck the sheet against a circumferential surface of the first conveyance belt; and 
 a second suction device configured to suck the sheet against a circumferential surface of the second conveyance belt, 
 wherein a friction coefficient of the circumferential surface of the second conveyance belt is made greater than a friction coefficient of the circumferential surface of the first conveyance belt to cause the second conveyance belt against which the sheet is sucked by the second suction device to pull and convey the sheet so that the sheet slides on the first conveyance belt against which the sheet is sucked by the first suction device when the leading edge of the sheet reaches the fixing nip. 
 
     
     
       17. The image forming apparatus according to  claim 13 , wherein, in response to a length of the conveyed sheet in the conveyance direction being shorter than a length from the transfer nip to an upstream end of the second conveyance belt and shorter than a length from a downstream end of the first conveyance belt to the fixing nip, the sheet advancing from the transfer nip to the fixing nip is conveyed with the circumferential speed of the first conveyance belt set to the first speed and the circumferential speed of the second conveyance belt set to the second speed and the sheet lying on the first and second conveyance belts is pulled and conveyed by the second conveyance belt while sliding on the first conveyance belt at a speed that is higher than the first speed. 
     
     
       18. The image forming apparatus according to  claim 13 , wherein, in response to a length of the conveyed sheet in the conveyance direction being longer than a length from the transfer nip to an upstream end of the second conveyance belt and longer than a length from a downstream end of the first conveyance belt to the fixing nip, the control unit performs control so that the circumferential speed of the second conveyance belt is changed to the second speed after a trailing edge of the sheet, conveyed by the first conveyance belt whose circumferential speed is set to the first speed and the second conveyance belt whose circumferential speed is set to the first speed, passes the transfer nip and before the leading edge of the sheet reaches the fixing nip, and,
 after the circumferential speed of the second conveyance belt is changed to the second speed, the leading edge of the sheet reaches the fixing nip with the sheet sucked against the second conveyance belt pulled and conveyed by the second conveyance belt while sliding on the first conveyance belt. 
 
     
     
       19. An image forming apparatus comprising:
 a transfer nip configured to transfer a toner image to a sheet conveyed at a transfer speed; 
 a first conveyance belt configured to convey the sheet to which the toner image is transferred at the transfer nip; 
 a second conveyance belt provided downstream of the first conveyance belt in a conveyance direction in which the sheet is conveyed and configured to convey the sheet; 
 a fixing nip provided downstream of the second conveyance belt in the conveyance direction and configured to fix the toner image to the sheet while conveying the sheet; 
 a suction unit provided for each of the first and second conveyance belts and configured to suck the sheet; and 
 a control unit configured to control an operation of the suction unit for sucking the sheet against each of the first and second conveyance belts, 
 wherein, in response to a length of the conveyed sheet in the conveyance direction being longer than a length from the transfer nip to an upstream end of the second conveyance belt, a circumferential speed of the first conveyance belt is set to a first speed at which the sheet advances from the transfer nip and a circumferential speed of the second conveyance belt is set to a second speed at which a leading edge of the sheet enters the fixing nip and which is higher than the first speed and the control unit controls the suction unit so that the sheet is sucked against the first conveyance belt and slides on the second conveyance belt in a state that the sheet to which the toner image is transferred by the transfer nip is being conveyed by the first conveyance belt and controls the suction unit to cause the second conveyance belt to pull and convey the sheet so that the sheet slides on the first conveyance belt after a trailing edge of the sheet, conveyed by the first conveyance belt whose circumferential speed is set to the first speed, passes the transfer nip. 
 
     
     
       20. The image forming apparatus according to  claim 19 , wherein the control unit controls the suction unit so that the sheet is sucked against the first conveyance belt and the sheet is not sucked against the second conveyance belt until a trailing edge of the conveyed sheet passes the transfer nip, and controls the suction unit so that the sheet is sucked against the second conveyance belt and the sheet is not sucked against the first conveyance belt after the trailing edge of the conveyed sheet passes the transfer nip and before the leading edge of the sheet reaches the fixing nip. 
     
     
       21. The image forming apparatus according to  claim 19 ,
 wherein the suction unit includes a first suction device configured to suck the sheet against the first conveyance belt and a second suction device configured to suck the sheet against the second conveyance belt, 
 wherein the first suction device includes a first fan and a first shutter configured to open and close between the first fan and the first conveyance belt, 
 wherein the second suction device includes a second fan and a second shutter configured to open and close between the second fan and the second conveyance belt, and 
 wherein the control unit opens and closes the first and second shutters to control the operation of the suction unit for sucking the sheet against the first and second conveyance belts. 
 
     
     
       22. The image forming apparatus according to  claim 19 , wherein, in response to a length of the conveyed sheet in the conveyance direction being longer than a length from the transfer nip to an upstream end of the second conveyance belt and longer than a length from a downstream end of the first conveyance belt to the fixing nip, the circumferential speed of the first conveyance belt is set to the first speed and the circumferential speed of the second conveyance belt is set to the second speed and the control unit controls the suction unit so that the sheet is sucked against the first conveyance belt and slides on the second conveyance belt in a state that the sheet to which the toner image is transferred by the transfer nip is being conveyed by the first conveyance belt and controls the suction unit to cause the second conveyance belt to pull and convey the sheet so that the sheet slides on the first conveyance belt when the leading edge of the sheet reaches the fixing nip.